Well, for what it's worth, my N900 screen doesn't flex in any noticable way, and it's totally flat (not going down), if you don't count the little raised section around the rim (which includes the NOKIA text).

As for sound, touchscreens will whine a little bit. Very high frequency sound. I can't hear it. The N800 does too, but I had to go to the the youngest member of our office (around 24) to find someone who could actually hear it, due to the frequency. I couldn't, but I had been told about the screen noise and thus the test.